User Agent: Mozilla/5.0 (Windows NT 5.1; rv:13.0) Gecko/20100101 Firefox/13.0.1 Build ID: 20120614114901 Steps to reproduce: Receiving messages when TB is first opened. There are many filters applied to incoming messages, most of which sort/move messages to different folders. Mostly this works ok, until at some point, it does not. There are a large number of accounts, some with a large number of messages and a large number of filters and folders involved. Actual results: I see "Folder cannot be found" error reported. Is erroneous -- the folder always exists. Message is not moved, and the folder indicated in the filter rule is blanked out and the filter rule is disabled. Not only that, but then several other messages after that also result in the same error report, are not moved, but their folders are not blanked out, but the filter rules are disabled. I then go through all my filters looking for those that are disabled, and reapply them manually (adding back the folder name for the one filter that was blanked out.) They always work, proving there is nothing wrong with the filter or the folder. Expected results: TB should have moved the messages without any errors, should not have blanked out the folder name in the filter, and should not have disabled any filters. Additional comments: This problem does not always happen. When it does, it happens for different accounts, different filters, different folders. Seems like when it happens for one filter/folder, then several filter/folders after that are also disabled (maybe handled in the same thread? Many are in the same account, but some in one or two other accounts.) After the initial reading of messages when TB is first opened, and the filters are re-enabled, it does not seem to happen again while TB continues running, but that could be because there are very few messages being read at one time. There are also times when I get a "Folder in use" message during filtering, but TB seems to recover from that situation without any problems. Again this happens when reading a large number of messages when TB is first opened, probably because two different filters move messages to the same folder, from two different accounts (I guess handled by two different threads). I suspect I could eliminate this problem if I could limit TB to just one thread, but I cannot see a way to do that. It's not a serious problem, else I would submit a separate bug report, but it might be a factor contributing to the "Folder not found" problem.

I did not initially report this with release 13.0, but I don't see this in the bug list for release 14.0 so I realized that it was not reported or fixed, so I am reporting it now. I will upgrade to release 14.0 shortly. Not expecting it to be fixed, but if it does not happen again for a week or so after upgrading, I may report back that it does not happen in release 14.0. Otherwise, unless a developer has reason to believe it was fixed, please consider this as still an active bug.

"Large number of accounts ... and ... filters" -- some more accurate numbers. Over 20 accounts. One account has approx. 330 filters, two accounts have approx. 100 filters, three others have between 50 and 90 filters. Most of the remaining accounts have 20 filters or less. Accounts with the most filters have the most messages. Usually the biggest account has 50 to 100 incoming messages when TB is first opened.

Yes, we believe this is fixed in bug 751562. Please upgrade to 14 and reopen this your bug if you still find it happening. Also a temporary workaround (until you upgrade) is to keep all of your folders expanded in the left folder pane.